1. Field of the Invention
The present invention relates to a communication system, in particular, to a communication system capable of determining, at a reception side of a communication circuit, whether a communication method connected is of optical communication or electric communication.
2. Description of the Related Art
As communication methods in communications performed via a cable between electronic devices, with which either of an optical signal or an electrical signal can be optionally selected for data transmission, the following methods are available:
(1) a method for connecting an electric communication cable 809 to electric connectors 803, 804 mounted on electronic devices 801, 802, using electric connectors 805, 806,
or a method for connecting an optical active cable that performs communication via an optical communication cable 810, using photoelectric conversion circuits incorporated in electric connectors 807, 808 provided at both ends thereof as illustrated in FIG. 8, or.
(2) a method for connecting an electric cable 923 to electric communication modules 915, 916 which can be attached to, and detached from, electric connectors 913, 914 mounted on electronic devices 911, 912, using electric connectors 917, 918,
or a method for connecting an optical communication cable 924 to photoelectric conversion modules 919, 920, using optical connectors 921, 922, as illustrated in FIG. 9.
In the method (2) above, a small form-factor pluggable (SFP) module is widely known.
In general, optical communication enables higher-speed and longer-distance communication than electric communication. Furthermore, waveform correction techniques for achieving higher-speed and longer-distance communication with electric communication have been developed although less effective than optical communication, wherein pre-emphasis and de-emphasis, and an equalizer are widely used. If a cable that is connected is known at the side of an electronic device, it is possible to select a communication speed and to determine whether or not to use a waveform correction method, whereby communication optimally utilizing the performance of each cable can be performed.
As a method for informing an electronic device of a selected communication method, electric communication or optical communication, a user may set the selected communication method in an electronic device, or an electronic device may automatically discriminate the selected communication method. Examples of such automatic discrimination by an electronic device include a configuration in which a terminal for discriminating the communication method selected is provided, as disclosed in Japanese Patent Application Laid-Open No. 2008-097040, and a configuration in which a transceiver (electric communication module or photoelectric conversion module) has therein information on corresponding communication standard including information on which of optical communication or electric communication is used and is configured to acquire information by using a dedicated communication path between an electronic device and the transceiver, as in an SFP transceiver.
In case where a system is established in which communication between electronic devices is performed by an inexpensive electric communication in a short-distance communication and a low-speed communication, whereas communication between electronic devices is performed by optical communication in a long-distance communication and high-speed communication, if both of the electronic devices that perform communication know the communication method before the start of communication, it is possible to determine whether or not a technique for correcting a waveform is to be used at the time of electric communication and to select a communication speed. However, adding a specific terminal for discriminating a communication method or a dedicated communication path increases costs. Because it is sufficient that the communication method is known before the start of communication, a terminal for communication used after the start of communication can be temporarily used. Furthermore, for electric communication, connection with AC coupling may be required to eliminate influence of a signal potential difference between electronic devices that perform communication.